This new investigator-initiated R01 proposal will test the efficacy of a promising and innovative intervention (Mighty Girls) to reduce teen Latina sexual behavior and pregnancy that was developed in a successful R15 feasibility trial. US teen pregnancy rates have declined but are still high among Latinas (49.4%), and the socioeconomic costs associated with these pregnancies are considerable; e.g., $3.2 billion lost tax revenue (2008 dollars) for decreased maternal educational attainment and productivity. Mighty Girls is designed to empower middle school Latinas with the skills they need to resist peer pressure that would otherwise accelerate their involvement in sexual behavior, allowing them to delay their initiation of intercourse and ultimately reduce their risk for teen pregnancy and sexually transmitted infections. Mighty Girls uses cutting-edge technology to create a novel, highly interactive, video game that provides a live, realistic simulation of peer pressure. Players talk directly with avatars as they would with peers in their everyday life. This intervention focuses on building communication skills for resistance that are derived from evidence-based programs that do not jeopardize friendships and incorporate Latino cultural values (Marianismo, Personalismo, Simpata) - skills valued by middle school Latinas. In addition to efficacy testing, the proposed interdisciplinary study evaluates (1) the intervention's cost- effectiveness and (2) influence of participant characteristics and how well the intervention was delivered (implementation quality) on study outcomes. The study will be conducted with seventh grade Latinas enrolled in 20 middle schools in Florida's Miami-Dade County, and followed into ninth grade, a period in which risky behaviors typically increase. Half of these schools will be randomly assigned to receive the Mighty Girls intervention and booster in addition to standard in-school sex education, while the other half will receive only standard in-school sex education. Data will be collected before and after the intervention, and at 2 (pre-booster), 12, and 24 months post-intervention using computerized surveys. Growth curve analysis will compare changes in peer resistance and sexual behavior observed in the two study conditions over the course of the study. Cost-effectiveness analyses will weigh the benefit of the intervention when added to current in-school sex education against its cost of delivery. This proposal addresses Healthy People 2020 objectives to reduce pre-term birth, increase the proportion of women who use contraception to plan pregnancy, and eliminate health disparities. It lays the foundation for a dissemination study of the Mighty Girls intervention that would have a major impact on the lives of vulnerable adolescents and reduce the personal and public burden of early sexual initiation. Study results would also advance the science of communications, health games, and computer engineering by generating data on human-avatar interactions and the use of highly interactive game technology to build health-related interpersonal skills.